This article assumes that you have basic knowledge of exposure, f-stops, shutter times and aperture values and how those last two relate. More on this subject will follow later.
How does your camera meter light?
Most, if not all modern cameras have built-in exposure meters. The photocell measures light reflected from the scene. The exposure meter tells the camera (you) how you should expose for a particular scene.
So, how does it know how you should set your exposure? Well, as a matter of fact, it doesn’t. The meter is calibrated to see the world in middle gray. A middle gray subject reflects 18% of the light that falls on it.
What is middle gray then, you may wonder. In terms of gray tones, it looks like this:
Try to compare this with subjects in your surroundings, to see which ones might have similar brightness.
How does this relate to your exposure meter?
Remember, the exposure meter is calibrated to see the world as being average, middle gray and it tells your camera that it should use exposure settings so that it would render the scene exactly middle gray.
Some scenes can be perfectly exposed like this, since usually things average out to be middle gray. But what happens when you decide to photograph a white wall and you would let your camera decide the perfect exposure? Since it is set to expose for middle gray, the final image would in fact look gray.
The same would happen if you would photograph a black wall, the camera would again expose for middle gray, and thus the final image would show a gray wall. This is why sometimes, exposures made on the beach, or in snow, can look too dark in the final images.
There are meters that are smarter than this, because they compare patterns in a scene with pre-programmed patterns in memory. For example, a bright top, and darker bottom half, which could in fact be a landscape scene. Darker foreground, brighter sky. Those meters are so-called matrix meters and quite usually they work very well.
But what if you want more or total control (and you do!)?
If your camera has a matrix meter, switch to center weighted mode, or spot mode. In center-weighted mode, your meter will take a reading of approximately 60 to 80 percent towards the central point of the view. If set to spot metering, this area will be as narrow as 1 to 5 percent.
Now you start to take control of the camera, instead of the camera controlling the exposure settings.
Now, if you want to photograph in the snow, you know you will have to expose more than what your meter tells you to do. If you have your camera set to make automatic exposures, you can set your exposure compensation to +1, or one stop. Try to find in your camera’s manual where to do this. It’s probably easier to just switch the exposure to full manual mode, if that is possible on your camera.
Choose your desired aperture (that would be easy for me, since I usually shoot my lenses with the largest opening possible). Let’s assume F/2.8. Now set your shutter time. Notice how the meter in your viewfinder starts to center towards the middle while you adjust your settings. If you would take a photo of snow covered scene for example, set your shutter time so, that the meter thinks the scene is over exposed by one to one and a half stop. (In some cameras this be indicated as a +, in some you will see small lines). There you have it. Take the shot, it will be perfectly exposed for the snow.
With some practice you can now also make photos of more complicated scenes, such as people against bright backgrounds. You could even choose to simply render someone as a silhouetted against a bright background, instead of having the person show normally exposed and the background overexposed.
Another example would be to perfectly expose for a person’s face. If this would be an average caucasian person, the skin tones would be approximately one stop above middle gray. We want the tones on the final image as we see them, not as middle gray. If you use center-weighted mode, place the face in the center of the frame and set your exposure. Make sure that the meter indicates approximately 1 stop over exposure. Reframe and make the shot.
If the subject has a dark(er) skin tone, you may want to make the exposure for one half to one stop below middle gray.
Here’s a nice little tip; if you are not sure how you should set your exposure for a particular scene, take a reading of the palm of your hand and add one stop. Then with the same exposure settings shoot the scene. Make sure your palm points towards the same light source that is used to light the scene. The sun, for example.
One nice thing to have amongst your equipment would be a gray card. It’s a card that reflects 18 percent of the light and can be used as a reference, or to take exposures from.